1. Field of the Invention
The present invention relates to a method of manufacturing a nano-size pitted structure having recesses and projections at a nano-scale pitch. More particularly, the present invention relates to a nano-structure and a method of manufacturing the nano-structure, which can be utilized as a recording medium, an optical functional device, etc.
2. Description of the Related Art
Hitherto, manufacturing a porous structure with the anodic oxidation process has received attention as a simple method for forming a structure having recesses and projections of from nanometer to micrometer sizes. The anodic oxidation process generally comprises the steps of immersing a workpiece in an acidic solution, and applying a voltage with the workpiece serving as an anode to cause phenomena of dissolution and oxidation of the workpiece surface at the same time, thereby forming a microscopic pitted structure. This microscopic structure is known as exhibiting regularity through self-assembly under certain conditions (such as voltage, temperature and kind of acid), and applications of the microscopic structure to various nano-devices, such as a recording medium and an optical functional device, are expected.
One practical example of the anodic oxidation process is anodic oxidation of Al. When a voltage is applied to a bath of an aqueous solution of oxalic acid, phosphoric acid, or sulfuric acid with Al serving as an anode, nanoholes surrounded by a barrier layer (alumina) are formed and a porous coating is obtained. While a similar phenomenon may also occur with another kind of metal, columnar pores (nanoholes) with the best straightness can be obtained using Al. The anodic oxidation of Al can form pores in a regular array in which the pore pitch is several tens nanometers (Appl. Phys. Lett., 72, 1173, 1998), or in an irregular array in which the average pore pitch is about 18 nm and the pore diameter is 8 nm (Japanese Patent Laid-Open No. 2000-195036).
Also, a pillar nano-structure has been fabricated by using, as a mold, a hole nano-structure formed by the anodic oxidation process (FIGS. 6A to 6D). In Japanese Patent Laid-Open No. 6-32675, Chemistry Letters, 621, 1990, and Jpn. J. Appl., 36, 7791, 1997, for example, a pillar structure is formed by using an alumina hole nano-structure 102 caused by the anodic oxidation of Al 101 as a mold, filling a material 104, such as a resin or a metal, in each of pores 103, and removing the alumina 102 mold. Further, it is proposed to perform a hole nano-structure 105 through stamping or filling with the pillar structure used as a die or a mold (J. Vac. Sci. Technol. B., 19(2), 569, 2001). Those proposals can produce a nanometer-size structure having a high aspect ratio, which cannot be obtained by manufacturing methods utilizing semiconductor processes. Here, the term “aspect ratio” represents y/x where x is the pore diameter and y is the pore depth.